1. Field of the Invention
The present invention relates to a level detector which is adapted to detect fluid levels continuously through the electrostatic capacitance variations between a pair of electrodes.
2. Description of the Prior Art
As set forth in Japanese Examined Patent Publication (KOKOKU) No. 62-18,004, there is a conventional level detector which detects the electrostatic capacitance variations between a pair of electrodes resulting from the liquid level variations in order to detect the liquid levels. This publication discloses that the electrostatic capacitance variations are detected through the frequency variations at the RC (Resistance-Capacitance) oscillating circuit in this conventional level detector.
However, when the electrostatic capacitance variations between a pair of the electrodes resulting from the liquid level variations are detected through the oscillating frequency variations, there is no linear relationship between the oscillating frequency and the electrostatic capacitance "C" because ".omega." is equal to "(LC).sup.-1/2 ". For instance, the signal voltages undergoing the F/V (Frequency/Voltage) conversion do not have a linear relationship with the liquid levels which virtually have a linear relationship with the electrostatic capacitance between the electrodes. Accordingly the subsequent signal processing is very complicated.
The present invention has been developed in view of the aforementioned drawback of the conventional level detector. It is therefore a primary object of the present invention to provide a level detector which exhibits a good linearity between fluid level variations and output signal voltage variations and which has a simple construction as well.
A level detector according to the present invention comprises:
a level detecting capacitor including a pair of continuously extended electrodes, the electrodes being separated by a predetermined distance and disposed at a predetermined position in a container adapted for accommodating a dielectric fluid having levels which are to be detected; PA1 an oscillating circuit adapted to apply an alternating voltage of a constant frequency to the level detecting capacitor by way of an impedance element, the impedance element being adapted to provide signal voltages; and PA1 an amplifying circuit adapted to amplify voltage drops at the impedance element so as to output a signal corresponding to the level of the dielectric fluid.
The present level detector operates as follows: The alternating voltage output by the oscillating circuit is applied to a series circuit constituted by the impedance element and the level detecting capacitor. Then, the voltage drop present at the impedance element is amplified by the amplifying circuit and is output as a signal which corresponds to the level of the dielectric fluid having levels which are to be detected. When the impedance of the impedance element is "Z," the electrostatic capacitance of the level detecting capacitor is "C," and the angular frequency of the alternating voltage is ".omega.," the linearity of the signal voltages output by the amplifying circuit can be improved by reducing ".omega.CZ" to less than 1 as much as possible, preferably to 0.1 or less.
As having been described so far, in the present level detector, the alternating voltage output by the oscillating circuit is applied to the series circuit constituted by the impedance element and the level detecting capacitor. The voltage drop present at the impedance element is then output as a signal which corresponds to the level of the dielectric fluid. As a result, the present level detector can output signal voltages of high linearity which depend on the fluid level variations with a simple circuit.
Further, the level detecting capacitor of the present level detector can be constructed as follows: The level detecting capacitor can include an outer cylinder and an inner cylinder which are disposed concentrically. The outer cylinder can be adapted to constitute an external electrode to which the alternating voltage is supplied from the oscillating circuit, and the inner cylinder can be disposed concentrically inside the outer cylinder and is adapted to constitute an inner electrode from which signal voltages are transmitted to the amplifying circuit.
Furthermore, the level detecting capacitor of the present level detector can be modified as follows: The level detecting capacitor can include an outer cylinder and an inner cylinder which are disposed concentrically, the outer cylinder being adapted to constitute an external electrode which is connected to a constant voltage line. The inner cylinder can be disposed concentrically inside the outer cylinder concentrically and is adapted to constitute an inner electrode which is connected to a terminal of the impedance element.